Nitrogen Heterocyclic Carbenes Research Articles

Low Valence Triel (I) Systems as Hydrogen Bond Acceptors and their Stability with Respect to Triel (III) Compounds.

A theoretical study of the complexes formed by carbene like Al(I), Ga(I), In(I) and Tl(I) compounds with hydrogen bond donors (HBD), XH (HCCH, HSH,HOH, HCN, HCl, HBr, HF, and HNC) have been carried out at MP2 computational level. The isolated triel(I) compounds show a negative region of the molecular electrostatic potential region associated with the triel atom suitable to interact with electron deficient groups. This region is associated to a lone pair based on the ELF analysis and to the location of the HOMO orbital. The complexes are similar to those found in nitrogen heterocyclic carbenes (NHC) with HBD. In addition, the oxidative addition reactions of those complexes to yield the corresponding valence III compounds have been characterized. The Al(III) compounds are much more stable than the corresponding Al(I) complexes. However, the stability of the triel(III) compounds decreases with the size of the triel atom and for the thallium derivatives, the Tl(I) complexes are more stable than the Tl(III) compounds in accordance with the number of the structures found in the CSD. The barrier of the TS connecting the triel(I) and triel(III) systems increases with the size of the triel atoms.

Application of DFT and TD-DFT on Langmuir Adsorption of Nitrogen and Sulfur Heterocycle Dopants on an Aluminum Surface Decorated with Magnesium and Silicon

In this study, we investigated the abilities of nitrogen and sulfur heterocyclic carbenes of benzotriazole, 2-mercaptobenzothiazole, 8-hydroxyquinoline, and 3-amino-1,2,4-triazole-5-thiol regarding adsorption on an Al-Mg-Si alloy toward corrosion inhibition of the surface. Al-Si(14), Al-Si(19), and Al-Si(21) in the Al-Mg-Si alloy surface with the highest fluctuation in the shielding tensors of the “NMR” spectrum generated by intra-atomic interaction directed us to the most influence in the neighbor atoms generated by interatomic reactions of N → Al, O → Al, and S → Al through the coating and adsorbing process of Langmuir adsorption. The values of various thermodynamic properties and dipole moments of benzotriazole, 2-mercaptobenzothiazole, 8-hydroxyquinoline, and 3-amino-1,2,4-triazole-5-thiol adsorbed on the Al-Mg-Si increased by enhancing the molecular weight of these compounds as well as the charge distribution between organic compounds (electron donor) and the alloy surface (electron acceptor). Finally, this research can build up our knowledge of the electronic structure, relative stability, and surface bonding of various metal alloy surfaces, metal-doped alloy nanosheets, and other dependent mechanisms such as heterogeneous catalysis, friction lubrication, and biological systems.

In Silico-DFT Investigation of Nanocluster Alloys of Al-(Mg, Ge, Sn) Coated by Nitrogen Heterocyclic Carbenes as Corrosion Inhibitors

In Silico-DFT Investigation of Nanocluster Alloys of Al-(Mg, Ge, Sn) Coated by Nitrogen Heterocyclic Carbenes as Corrosion Inhibitors

Application of Nitrogen Heterocyclic Carbenes in Organocatalysis

Background: During the last two decades, horizon of research in the field of Nitrogen Heterocyclic Carbenes (NHC) has widened remarkably. NHCs have emerged as ubiquitous species having applications in a broad range of fields, including organocatalysis and organometallic chemistry. The NHC-induced non-asymmetric catalysis has turned out to be a fruitful area of research in recent years. Methods: By manipulating structural features and selecting appropriate substituent groups, it has been possible to control the kinetic and thermodynamic stability of a wide range of NHCs, which can be tolerant to a variety of functional groups and can be used under mild conditions. NHCs are produced by different methods, such as deprotonation of N-alkylhetrocyclic salt, transmetallation, decarboxylation and electrochemical reduction. Results: The NHCs have been used successfully as catalysts for a wide range of reactions making a large number of building blocks and other useful compounds accessible. Some of these reactions are: benzoin condensation, Stetter reaction, Michael reaction, esterification, activation of esters, activation of isocyanides, polymerization, different cycloaddition reactions, isomerization, etc. The present review includes all these examples published during the last 10 years, i.e., from 2010 till date. Conclusion: The NHCs have emerged as versatile and powerful organocatalysts in synthetic organic chemistry. They provide the synthetic strategy which does not burden the environment with metal pollutants and thus fits in the Green Chemistry.

Binding indirect greenhouse gases OCS and CS2 by nitrogen heterocyclic carbenes (NHCs).

Carbon disulfide (CS2) and carbonyl sulfide (OCS) are indirect greenhouse gases that can be effectively trapped by classical, abnormal and remote nitrogen heterocyclic carbenes (NHCs), according to high level ab initio calculations. The process is described through a reaction profile involving two minima, a non-covalent complex and a covalently bound product, connected by a single transition state. Both CS2 and OCS react towards NHCs in a similar way, forming a new C-C bond and leading to very stable products with feasible barriers in many cases, although they vary significantly depending on the NHC structure. The barriers are larger than those reported for CO2, oscillating from barrierless processes up to a maximum of 57.9 kJ mol-1, whereas the products are more stabilized than those incorporating CO2. The lowest barriers for the CS2 + NHC reactions correspond to the largest C-C distances in the products, unlike the CO2 case. Remarkably, the most favored reactions, which are those involving a remote NHC, do not exhibit the highest interaction energies at the TS, but low distortion energy values of the OCS/CS2 moieties. The decomposition of the interaction energy allowed to confirmed that in fact the remote carbene is the less favored one in terms of the electrostatic and exchange terms. Substitution in CO2 of O by a more polarizable atom such as S have a great influence on the balance between the steric plus orbital interaction and the kinetic energy terms, thus making the products between NHCs and OCS/CS2 more stable. Both OCS and CS2 become better charge acceptors than CO2 on going from the starting complexes to the products.

Carbon-Carbon Bonding between Nitrogen Heterocyclic Carbenes and CO2.

Ab initio MP2/aug'-cc-pVTZ calculations were performed to identify equilibrium complexes and molecules and the transition structures that interconvert them, on the potential energy surfaces of a series of seven binary systems that have nitrogen heterocyclic carbenes (NHCs) as the electron-pair donors to CO2. Seven of the NHCs form complexes stabilized by C···C tetrel bonds, and six of these seven are also stabilized by a secondary interaction between an O of CO2 and the adjacent N-H group of the carbene. Six of the seven NHCs also form stable molecules with C-C covalent bonds, and with one exception, these molecules have binding energies that are significantly greater than the binding energies of the complexes. Charge-transfer stabilizes all of the NHC:CO2 complexes and occurs from the C lone pair of the carbene to the CO2 molecule. The six complexes that have secondary stabilizing interactions are also stabilized by back-donation of charge from the O to the adjacent N-H group of the carbene. Transition structures present barriers to the interconversion of complexes and molecules. With one exception, the barrier for converting a molecule to a complex is much greater than the barrier for the reverse reaction. Atoms in Molecules bonding parameters, shifts of IR C-O stretching and O-C-O bending frequencies, changes in NMR 13C chemical shieldings, and changes in C-C and C-O coupling constants as 1tJ(C-C) and J(C-O) for complexes and transition structures become 1J(C-C) and 2J(C-O) for molecules, are all consistent with the changing nature of the C···C tetrel bond in the complex through the transition state to a covalent C-C bond in the molecule.

Trapping CO2 by Adduct Formation with Nitrogen Heterocyclic Carbenes (NHCs): A Theoretical Study.

Carbon dioxide can form compounds with nitrogen heterocyclic carbenes (NHCs) based on azoles through noncovalent interactions or by covalent bonding. A narrow dependence on the carbene structure has been observed for the preference for one or the other type of bonding, as revealed by a series of physicochemical descriptors. In our survey, a set of NHCs based on the azole family (three classical, three abnormal, and one remote) was shown to bind CO2 at the accurate G4MP2 computational level. In most cases, exothermic reaction profiles towards the covalently bound form were found, which reached stabilization enthalpies of up to -77 kJ mol-1 for the remote carbene case. Both noncovalent and covalent minima and the corresponding transition state that connects them have been identified as stationary points along the reaction coordinate.

Oxidative degradation of the organometallic iron(II) complex [Fe{bis[3-(pyridin-2-yl)-1H-imidazol-1-yl]methane}(MeCN)(PMe3)](PF6)2: structure of the ligand decomposition product trapped via coordination to iron(II).

Iron is of interest as a catalyst because of its established use in the Haber-Bosch process and because of its high abundance and low toxicity. Nitrogen-heterocyclic carbenes (NHC) are important ligands in homogeneous catalysis and iron-NHC complexes have attracted increasing attention in recent years but still face problems in terms of stability under oxidative conditions. The structure of the iron(II) complex [1,1'-bis(pyridin-2-yl)-2,2-bi(1H-imidazole)-κN(3)][3,3'-bis(pyridin-2-yl-κN)-1,1'-methanediylbi(1H-imidazol-2-yl-κC(2))](trimethylphosphane-κP)iron(II) bis(hexafluoridophosphate), [Fe(C17H14N6)(C16H12N6)(C3H9P)](PF6)2, features coordination by an organic decomposition product of a tetradentate NHC ligand in an axial position. The decomposition product, a C-C-coupled biimidazole, is trapped by coordination to still-intact iron(II) complexes. Insights into the structural features of the organic decomposition products might help to improve the stability of oxidation catalysts under harsh conditions.

ChemInform Abstract: Metal‐Free Arylation of Benzene and Pyridine Promoted by Amino‐Linked Nitrogen Heterocyclic Carbenes.

AbstractAn amino‐linked nitrogen heterocyclic carbene mediates the carbon—carbon coupling reaction through direct C—H functionalization of benzene (II) and pyridine (IV) with aryl iodides (I) in the absence of a metal catalyst.

Metal-free arylation of benzene and pyridine promoted by amino-linked nitrogen heterocyclic carbenes

An amino-linked nitrogen heterocyclic carbene (amino-NHC), 1-tBu, has been shown to mediate carbon-carbon coupling through the direct C-H functionalization of benzene and pyridine in the absence of a metal catalyst. Using EPR, the first spectroscopic evidence corroborating the single electron transfer mechanism for the metal-free carbon-carbon coupling manifold, as reported by others, is introduced.

Synthesis of 2-Keto-imidazoles Utilizing N-Arylamino-Substituted N-Heterocyclic Carbenes

A new method for the synthesis of 2-aroyl-, 2-heteroaroyl-, and 2-cinnamoyl-substituted imidazoles in very good yields has been developed. The reaction employs novel nitrogen heterocyclic carbenes (NHCs), namely, N-arylamino-substituted NHCs, formed in situ from the corresponding imidazolium salts, and subsequent reaction with aromatic, heteroaromatic, and cinnamic aldehydes without utilizing transition metals or expensive specialized catalysts.

Gold(I) Complexes with Hydrogen-Bond Supported Heterocyclic Carbenes as Active Catalysts in Reactions of 1,6-Enynes

Complexes [AuCl{C(NHR)(NHPy-2)}] (Py-2 ) 2-pyridyl; R ) Me, tBu, nBu, iPr, nheptyl) have been prepared in amodular way from [AuCl(CNPy-2)]. The carbene moiety has a hydrogen-bond supported heterocyclic structure similar to the nitrogen heterocyclic carbenes in the solid state, and in CH2Cl2 or acetone solution, which is open in the presence of MeOH. The compounds are good catalysts for the skeletal rearrangement of enynes, and for the methoxycyclization of enynes. In contrast, the complexes [AuCl{C(NHR)(NHPy-4)}] are scarcely active due to the blocking effect of the coordination position required for the catalysis by the nitrogen of the NHPy-4 group.

On the interactions of N, N′-bismesitylimidazolin-2-yl and alcohols

The interaction of N, N′-bismesitylimidazolin-2-yl (IMes) with alcohols is discussed. NMR spectroscopy and X-ray crystallography were used to examine the influence of solvent and alcohol on this interaction. The stabilizing effect of these interactions may be used for the storage of nitrogen–heterocyclic carbenes (NHCs) since removal of the volatiles liberates the NHC.